The price of street gas being what it is, even the most hard-core hot rodder must grasp the consequences. Certainly, they can curtail driving to reduce fuel cost, which often precludes traditional destinations and may even cut into appearances at the local drive-in. There is a sensible way to combat this issue, even if it only means a few miles per gallon. Overdriven top gears have been around for many years, mostly at the behest of the OE manufacturers-see, they actually did learn something from the fuel-blank debacles of '73-79. Overdrive automatics became standard equipment a quarter-century ago. It's about time hot rodders got hip to this worthy upgrade. We can't believe anyone would still want to "cruise" with straight 4.56:1 gears.
The abuse the engine suffers at 3,500 droning rpm is a lot more worrisome than one gliding along the high-speed at a sedate two grand. Think how much quieter things will become. Think how much you'll reduce the size of your environmental footprint. The cruel fact is that as much as hot rodders have historically scoffed at the rules and the socially accepted norms of the period, we have to start playing the establishment game, at least to some extent. Here's one way to do it.
Though the notion seems simple enough-replace a transmission with a similar, albeit electronically controlled, one-all is not what it appears to be. Will the bellhousing attachment holes correspond to the ones on the back of the block? Will the driveshaft bolt back in? Is the hardware compatible? Before you begin, seriously consider using new fasteners available from any GM dealer or aftermarket source.
What Is It
How to tell the difference between factory overdrive transmissions.
The information is here; the rest is up to you.
Contingent on your choice.
The vital link: The top driveshaft is an aluminum unit used for Powerglide, Turbo 350, 700R4, and 2004R with a 27-spline yoke. The second unit, also with a 27-spline yoke, is for 4L60E and 4L65E. The bottom prop shaft is a Turbo 400 or 4L80E type with a 32-spline output shaft. In many cases, the transmission you're swapping in for a Turbo 350 or Turbo 400 will have a longer tailshaft than the OE unit, requiring the services of a competent driveshaft shop to shorten (and rebalance) it the appropriate amount (3 inches or less).
Bellhousing Bolt Pattern Identification
Aod Spotter's Guide
Screenshots courtesy of Monster Transmission & Performance.